1. Field of the Invention
The invention relates to a method of driving an on-demand multinozzle ink jet head which uses a piezoelectric element, and particularly to a method of driving an on-demand multinozzle ink jet head in which a large number of nozzles are integrated at high density.
2. Description of the Related Art
In some on-demand ink jet heads, an ink pressurizing chamber is configured by opposing a plate in which a plurality of orifices are formed to a diaphragm which is to be elastically deformed by a piezoelectric element, and ink is subjected to suction and pressurization by expansion and contraction of the piezoelectric element, thereby ejecting an ink drop through a nozzle. In such an on-demand ink jet head, it is necessary to satisfactorily couple the piezoelectric element to the diaphragm, so that the displacement of the piezoelectric element is efficiently transmitted to the pressurizing chamber.
For example, Japanese Patent Unexamined Publication No. Sho 58-119872 proposes a technique in which a coupling member is inserted between a diaphragm and a piezoelectric element. Japanese Patent Unexamined Publication No. Hei 6-143573 proposes a technique in which an island portion is formed in a diaphragm at a position where a piezoelectric element is in contact with the diaphragm. Both the techniques are intended to efficiently transmit the driving force of the piezoelectric element to the diaphragm, and used for reducing variations among nozzles during a production process.
A conventional nozzle driving method will be described with reference to an exemplary case in which nozzles are driven by using a rectangular wave shown in FIG. 7.
FIG. 8 shows an example of an ink drop velocity obtained in experiments conducted by the inventors in which one prior art nozzle produced in accordance with appropriate specifications was used and the pulse width was changed while maintaining the voltage at a constant level. In the case of this nozzle, the ink drop velocity exhibits the maximum (peak) in the vicinity of 4.5 .mu.s. The peak corresponds to a resonance point of the Helmholtz resonance vibration of the nozzle which is determined depending on the sizes, the materials, the physical properties, and the like of the ink passage system, the diaphragm, the piezoelectric element, etc. If the nozzle is driven by the pulse width at the peak position (4.5 .mu.s), a high ink drop velocity can be obtained at a low voltage. Accordingly, such a prior art nozzle driving method has an advantage in that a desired ink drop velocity can be obtained by reduced power consumption.
The driving in the invention is performed in accordance with the following manner.
A condition in which the meniscus of a nozzle is neutral is regarded as an initial condition. A driving waveform is applied to a piezoelectric element. The meniscus is moved backward from an orifice (the direction from the orifice toward a printing sheet is defined to be forward, and the opposite direction is defined to be backward), so that ink is sucked from an ink tank via a restrictor. Thereafter, a pressure is applied to the ink-by a diaphragm so as to eject the ink to the outside from the orifice. In the case of a pulse of a rectangular wave, at the last timing of one pulse, an operation in which the diaphragm presses the ink and a pressure is applied thereto is performed. If the phase in which the meniscus is moved forward is established at this timing, the ink velocity is the maximum.
In view of this point, in the driving in the invention, it is presumed that a pulse width at which the drop velocity is the peak in FIG. 8 is a half of the period of the Helmholtz resonance vibration. In other words, the period of the Helmholtz resonance vibration is obtained as a doubled value (9 .mu.s) of the pulse width (4.5 .mu.s) at the peak in FIG. 8. When a nozzle is appropriately modeled as a vibration system, the Helmholtz resonance vibration period can be obtained by calculation.
For the purpose of increasing the printing speed, a multinozzle ink jet head in which a plurality of nozzles are integrated is the most suitable. In such a multinozzle ink jet head, however, variations may be caused in nozzle characteristics because of various reasons.
In the production of a conventional multinozzle ink jet head, for example, several thin plates or a dozen of thin plates of orifices and the like which are configured by an etched thin plate of stainless steel having a thickness in the range of several micrometers to several hundreds of micrometers or thin nickel plates formed by electroforming are often used as a member constituting a pressurizing chamber. Nozzles are formed by adhesion or metal bonding of these thin plates. In the case of adhesion, an adhesive agent between the plates must be cured, and, in the case of metal bonding, a metal which serves as a coupling member between the plates must be melted. Therefore, a heating process is required in both the cases. If different kinds of metals are combined by heating, there occurs residual heat distortion caused by a difference between coefficients of thermal expansion of the members. This causes the head to slightly warp.
In such nozzles, the pulse width at the resonance point is slightly varied from nozzle to nozzle. Even if nozzles have substantially the same resonance point, a difference is produced in peak values. On the other hand, in such a multinozzle head, when all nozzles are driven by a pulse width at the above-mentioned peak value or in the vicinity thereof and a constant voltage, the driving voltage can be lowered, but the drop velocity may be largely varied from nozzle to nozzle. This produces a large obstruction for higher printing quality.